Sexism that I've noticed has been very subtle. I work with some very intelligent people, so I can't imagine anyone is overtly sexist (ie. thinking that women can't do this job, would not hire a person simply based on gender etc.), but still, there are problems. It's hard to write about sexism as a guy without feeling like you're coming off as sexist, but hey, I probably am. Also, it's hard to write about things without generalizing, so I don't think that these apply equally to all organizations, or all members of either gender, but as a population (of people and of companies), my thoughts are:

For one, I think that recruiting is very masculine. "Solve impossible problems! Be a rockstar! Join the most hardcore developers in the world!". It works, but I think that men find a lot more appeal in it than women do. I think that things like mentoring and career growth are also important - not that that detracts from the end result of solving really hard problems, but I think that it presents it to a more growth oriented way. So I think that especially college recruiting should still focus on the fun, hard engineering aspect of the job, it should also focus on the growth side. We'll train you, you'll work with people - we want smart people, we'll make you into an awesome developer.

Aggressiveness. If I want a raise, I'll go and ask for a raise. If I want something changed, I'll speak up about it. If I'm not happy with things, I'll quit and find another job. I think that this trait is more common in men, but it has certainly led to my success - pay quickly diverges with this strategy.

Then there's the social aspect. Within the office, I think that everyone talks with everyone else on pretty equal footing. That said, while I'd be totally comfortable asking one of my male colleagues if he wanted to grab a beer after work, I'd be at least more hesitant to do so with a female colleague so as not to make them uncomfortable (being married makes this a little bit less awkward for me) and I think that's not unique. People don't want to come off as flirting with a co-worker, or make them uncomfortable, and so they don't go out after work except in a group (and have no other recourse when they are the only female developer in a staff of 50). The people who do go out though end up talking about work, and end up solving problems that they wouldn't have otherwise. They also get more opportunity to move between projects as their friends move around within the company.

Then there's the awful problem of low numbers. If you meet a crummy male developer, well, he's a crummy developer. If you meet a bad female developer, she's probably a significant portion of all female developers that you've met - and so at least subconsciously, I think that for many the association becomes that female developers are bad (since hey, 33.3% of all female developers you've worked with aren't that great).

Those are the main things I think. I think that all of these can be fixed, but I think that it takes conscious effort on the part of the company and the staff to get around them. I think at the heart of it is a bootstrapping problem - there are too few female developers because there are too few female developers, and as that changes culturally things will get better. We can definitely speed it up though.

As an aside, I talked to a professor once about sexism in the computer science faculty, and he said something really interesting: while the top few students in the class are typically men due to the sheer number of people (if 90% of students are men, 10% are women, by pure statistics the top 1% will mostly be men), very rarely are women in the bottom 75% of the class. He attributed this to a lack of ego - there's a real masculine competitiveness to the computer science faculty (the goal to be "hardcore"), which women were generally excluded from. Without this ego drive, they weren't too proud to learn.